Apparatus for reducing evaporator resistance in a heat pipe

ABSTRACT

An arrangement is provided for cooling a heat-generating device (e.g., an integrated circuit chip) in a system such as a laptop computer. The arrangement includes a heat pipe having an evaporator made of porous materials with substantially uniformly sized pores and a wick made of finely porous material. The evaporator is thinner, on average, than the wick.

BACKGROUND

1. Field

The present invention relates generally to liquid cooling systems and,more specifically, to heat pipes for dissipating heat generated byintegrated circuits.

2. Description

As integrated circuits (e.g., central processing units (CPUs) in acomputer system) become denser, components inside an integrated circuitchip are drawing more power and thus generating more heat. Variouscooling systems have been used to dissipate heat generated by integratedcircuit chips, for example within personal computers, mobile computers,or similar electrical devices.

A heat pipe is a commonly used in a cooling system to dissipate heatgenerated by integrated circuits, especially CPUs, inside a computersystem. A heat pipe may include an evaporator section and a condensersection. Heat may be transferred from the evaporator section to thecondenser section through vapor generated by an evaporator in theevaporator section by evaporating a liquid coolant. The vapor maycondense back to liquid form at the condenser section through a heatexchanger coupled to the heat pipe. A heat pipe may also include a wickto act as a pump to bring the liquid coolant back from the condensersection to the evaporator section. The evaporator may again evaporatethe liquid coolant, drawing to the evaporator section by the wick, whenheated by the heat generated by an integrated circuit chip. The heattransfer rate from the integrated circuit chip into the liquid coolantin the evaporator section depends on evaporation resistance. The lowerthe evaporation resistance is, the higher the heat transfer rate is.Thus, it is desirable to reduce the evaporation resistance wheneverpossible.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present invention will becomeapparent from the following detailed description of the presentinvention in which:

FIG. 1 illustrates an exemplary block diagram of a computer system whichmay be utilized to implement embodiments of the present invention;

FIG. 2 illustrates an exemplary block diagram of a heat pipe along witha heat exchanger;

FIG. 3 is a block diagram illustrating an example of a heat pipe,according to one embodiment of the present invention;

FIG. 4 is an internal top view of an example implementation of a heatpipe, according to one embodiment of the present invention; and

FIG. 5 is a side view of the heat pipe whose top view is shown in FIG.4, according to one embodiment of the present invention.

DETAILED DESCRIPTION

Evaporation resistance in a heat pipe may depend on theevaporation/boiling process in the evaporator section of the heat pipeand also on the ability to wet the evaporator section, which in turndepends on the structure of the evaporator and the wick. Theevaporation/boiling process depends negatively on the thickness of theevaporator structure, while the ability to wet the evaporator section bythe wick through its capillary pumping depends positively on thethickness of the wick structure. The evaporator and the wick arenormally made of porous materials. According to an embodiment of thepresent invention, a thin porous evaporator structure made of asubstantially uniform pore sized porous material and a thick wickstructure made of a finely porous material may be used to help reduceevaporation resistance. A thin porous structure for the evaporator helpsreduce thermal conduction losses. A substantially uniform pore sizedmaterial used for the evaporator allows for minimum vapor flowresistance, small and substantially uniform spaces for vapor generation,and substantially uniform wicking in the evaporator section. All ofthese lead to a low evaporation resistance. On the other hand, a thickwick structure made of finely porous material may improve capillarypumping of the wick and thus result in the better ability of the wick towet the evaporator section, which helps reduce the evaporationresistance.

Reference in the specification to “one embodiment” or “an embodiment” ofthe present invention means that a particular feature, structure orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrase “in one embodiment” appearing in variousplaces throughout the specification are not necessarily all referring tothe same embodiment.

FIG. 1 illustrates an exemplary block diagram of a computer system whichmay be utilized to implement embodiments of the present invention.Although not shown, the computer system is envisioned to receiveelectrical power from a direct current (DC) source (e.g., a battery)and/or from an alternating current (AC) source (e.g., by connecting toan electrical outlet). The computer system comprises a centralprocessing unit (CPU) or processor 110 coupled to a bus 115. For oneembodiment, the processor 110 may be a processor in the Pentium® familyof processors including, for example, Pentium® 4 processors, Intel'sXScale® processor, Intel's Pentium® M processors, etc., available fromIntel Corporation. Alternatively, other processors from othermanufacturers may also be used.

The computer system as shown in FIG. 1 may also include a chipset 120coupled to the bus 115. The chipset 120 may include a memory control hub(MCH) 130 and an input/output control hub (ICH) 140. The MCH 130 mayinclude a memory controller 132 that is coupled to a main memory 150.The main memory 150 may store data and sequences of instructions thatare executed by the processor 110 or any other device included in thesystem. For one embodiment, the main memory 150 may include one or moreof dynamic random access memory (DRAM), read-only memory (RAM), FLASHmemory, etc. The MCH 130 may also include a graphics interface 134coupled to a graphics accelerator 160. The graphics interface 134 may becoupled to the graphics accelerator 160 via an accelerated graphics port(AGP) that operates according to an AGP Specification Revision 2.0interface developed by the Intel Corporation. A display (not shown) maybe coupled to the graphics interface 134.

The MCH 130 may be coupled to the ICH 140 via a hub interface. The ICH140 provides an interface to input/output (I/O) devices within thecomputer system. The ICH 140 may be coupled to a Peripheral ComponentInterconnect (PCI) bus. The ICH 140 may include a PCI bridge 145 thatprovides an interface to a PCI bus 170. The PCI Bridge 145 may provide adata path between the CPU 110 and peripheral devices such as, forexample, an audio device 180 and a disk drive 190. Although not shown,other devices may also be coupled to the PCI bus 170 and the ICH 140.

The CPU 110, the chipset 120, and other devices in the computer systemas shown in FIG. 1 may use a heat pipe to dissipate heat generated bythem.

FIG. 2 illustrates an exemplary block diagram of a heat pipe along witha heat exchanger. The heat pipe 210 comprises a sealed container whoseinner surfaces have a capillary material that forms the wick (not shownin FIG. 2). One end (212) of the heat pipe may be coupled to aheat-generating device 220 (e.g., a processor). The heat generated bythe device 220 transfers to the working fluid inside the heat pipe byevaporating the working fluid. The section inside the heat pipe (nearthe heat-generating device) where the working fluid is evaporated isalso called an evaporator section. The pressure difference inside theheat pipe may help transport the vapor of the working fluid from theevaporator section to the other end of the heat pipe, which may becoupled to a heat exchanger 230. The heat exchanger 230 transfers theheat from the vapor to ambient air so that the vapor may condense backto liquid. The section inside the heat pipe (near the heat exchanger)where the vapor condenses is also called a condenser section. After thevapor condenses, the liquid then moves to the evaporator section withthe help of the wick. This process continues so long as theheat-generating device generates enough heat to evaporate the workingfluid in the evaporator section.

The container is leak-proof so that it can isolate the inside workingfluid from the outside environment. The container maintains the pressuredifferential across its walls, and enables transfer of heat to takeplace from and into the working fluid. Selection of the containermaterial depends on many factors such as compatibility (both withworking fluid and external environment), strength to weight ratio,thermal conductivity, ease of fabrication, and porosity. The materialshould be non-porous to prevent the diffusion of the vapor of theworking fluid. A high thermal conductivity ensures minimum temperaturedrop between the heat source and the wick. Although it is shown as arectangular “L” shape in FIG. 2, the container can be any other shape(e.g., a straight or “L” shape cylinder) and any size so long as the end212 can be made fit with a heat-generating device and the other end canbe made fit to a heat exchanger.

It is desirable that the working fluid can be evaporated by aheat-generating device. In one embodiment, the working fluid may bewater, alcohol, glycol, an inert liquid, combinations thereof,surfactants, mixtures thereof, and the like. A high value of surfacetension may be desirable in order to enable the heat pipe to operateagainst gravity and to generate a high capillary driving force. Inaddition to high surface tension, it is also desirable for the workingfluid to wet the wick and the container material. A high latent heat ofvaporization is desirable in order to transfer large amounts of heatwith minimum fluid flow, and hence to maintain low pressure drops withinthe heat pipe. The thermal conductivity of the working fluid shouldpreferably be high in order to minimize the radial temperature gradientand to reduce the possibility of nucleate boiling at the wick or wallsurface. The resistance to fluid flow will be minimized by choosingfluids with low values of vapor and liquid viscosities.

The capillary structure or the wick over the inner surfaces (not shownin FIG. 2) of the container may be a porous structure made of materialslike steel, aluminum, nickel or copper in various ranges of pore sizes,fabricated using metal foams. Fibrous materials, such as ceramics andcarbon fiber filaments, may also be used. The main purpose of the wickis to generate capillary pressure to transport the working fluid fromthe condenser section to the evaporator section. It should also be ableto distribute the liquid around the evaporator section to any area whereheat is likely to be received by the heat pipe. The selection of thewick for a heat pipe depends on many factors. The maximum capillary headgenerated by a wick increases with a decrease in pore size. Anotherfeature of the wick is its thickness. The heat transport capability ofthe heat pipe may be raised by increasing the wick thickness. Theoverall thermal resistance in the evaporator section also depends on theconductivity of the working fluid in the wick. Other necessaryproperties of the wick are compatibility with the working fluid andwettability. Types of commonly used wick comprise sintered powder,grooved tube, and screen mesh.

Although it is desirable that the working liquid can be evaporated bythe heat from a heat-generating device, in one embodiment, there may beno evaporation process or only a partial evaporation process. The colderliquid may move from one end, which is coupled to a heat exchanger tothe other end, which is coupled to a heat-generating device, and isheated there to become hotter liquid (or hotter liquid and vapormixture), which then moves back to the colder end.

FIG. 3 is a block diagram illustrating an example of a heat pipe. Anattach block 310 may attach the heat pipe 330 to a heat-generatingdevice 320. In one embodiment, the attach block may be a part of theheat pipe as shown in FIG. 2. In another embodiment, the attach blockmay be coupled to the heat pipe so that the attach block may efficientlytransfer heat from the heat-generating device 320 to the evaporatorsection of the heat pipe 330. For example, when the container of theheat pipe is a cylinder and a heat-generating device has a flat surface,an attach block may be used to attach the heat pipe to theheat-generating device. Alternatively, the cylinder-shaped heat pipe maybe made to have a flat end to serve as the attach block.

Inside the heat pipe 330 there may be a vapor area 332 and a wick 334.The vapor generated in the evaporator section may transport through thevapor area 332 to the colder end of the heat pipe because of pressuredifference between the colder end and the hotter end where theevaporator section locates. Opposite to the end where the evaporatorsection is located, the other end of the heat pipe may be coupled to aheat exchanger 340. The heat exchanger may comprise a fan 342 and aplurality of fins 344. The fan 342 helps increase air circulation togenerate higher air flow so that heat carried by the vapor inside theheat pipe may be dissipated faster. The plurality of fins 344 increasethe contact area between the heat exchanger and the ambient air toimprove efficiency of heat transfer from the vapor inside the heat pipeto the ambient air. When the vapor transfers heat inside it to theambient air through the heat exchanger, the vapor condenses and returnsto the liquid state. The liquid then moves back to the evaporatorsection (not shown in FIG. 3) through capillary actions of the wick 334.

FIG. 4 is an internal top view of an example implementation of a heatpipe, according to one embodiment of the present invention. The heatpipe may include outer walls 410, a wick 420, and an evaporator 430.Although not explicitly illustrated in FIG. 4, the heat pipe may alsoinclude a liquid coolant. An evaporator section of the heat pipe may belocated near evaporator 430, and a condenser section of heat pipe may bespaced apart from evaporator 430 (e.g., including the far end of heatpipe). The heat pipe may be of any size. Outer walls 410 may enclosewick 420, evaporator 430, and the coolant. Outer walls 410 may contact aheat-generating device (e.g., an integrated circuit chip), and they mayinclude a highly thermally conductive material, such as copper oranother material. Outer walls 410 may be formed in a roughly rectangularshape, as illustrated in FIG. 1, or any other geometry that facilitatesaccess to evaporator 430 by the coolant and facilitates contact betweenouter walls and surfaces of the heat-generating device. Outer walls mayalso be formed to prevent the escape of vapor or liquid.

Wick 420 may include a porous material (e.g., sintered spherical copperparticles, sintered metal powder, a fiber material, or a screenmaterial, or a mixture of any of the above) that covers an inner surfaceof the heat pipe, except for the area occupied by evaporator 430. Wick420 may, by virtue of its porous structure, bring coolant from thecondenser section of heat pipe to the evaporator section. In thismanner, wick 420 may act to hydrate evaporator 430. In otherimplementations, wick 420 may include axial grooves that act to bringcoolant from the condenser section of heat pipe to the evaporatorsection. Other types of homogenous structures for wick 420 may includean open annular structure, an open artery structure, and/or an integralartery structure. In still other implementations, various compositestructures may be used for wick 420 that may include one or more of thehomogeneous structures noted above (e.g., sintered particles, screen,fibers, grooves, etc.).

Wick 420 may be made of a thick layer of finely porous material. Theporous material may include particles that have an average diameter in arange from 2 μm to approximately 100 μm. The average thickness of wickmay range from about 0.5 mm to approximately 1 mm. Wick 420 may bedesigned to have a relatively high capillary pumping efficiency tohydrate evaporator 430. Typically, the smaller the pore size is, thebetter the capillary pumping is. Good capillary pumping of the wickhelps improve the ability of the wick to bring the liquid coolant fromthe condenser section to the evaporator section and the ability of thewick to wet the evaporator, which may in turn improve theevaporation/boiling process in the evaporation section and thus reduceevaporation resistance.

Evaporator 430 may include a porous material (e.g., spherical metalparticles of various sizes sintered onto the inner surface of outer wall410) that roughly corresponds in area and orientation to a top surfaceof the heat-generating device to be cooled. In one embodiment, theporous material in evaporator 430 may be the same as the porous materialin wick 420. In another embodiment, the evaporator may use differentporous material from that used in the wick. Evaporator 430, by virtue ofits geometry and material, may have a relatively low thermal resistance.Because of the evaporator's low thermal resistance, the material of wick420 may have a somewhat higher thermal resistance without adverselyaffecting the heat transfer efficiency of heat pipe.

The porous material of evaporator 430 may be formed on the inner surfaceof the heat pipe with an average thickness which may be less than theaverage thickness of wick 420. A thin structure for the evaporator mayhelp reduce thermal conduction losses. The porous material of evaporator430 may include, for example, copper particles whose average size isgreater than the average particle size of the wick to improve vaporflow. A substantially uniform pore sized material may be used for theevaporator to reduce vapor flow resistance, to create small andsubstantially uniform spaces for vapor generation, and to formsubstantially uniform wicking in the evaporation section. In oneembodiment, evaporator 430 may comprise one to two layers of porousmaterial with substantially uniformly sized pores. The average diameterof the particles in the evaporator may be from approximately 25 μm to150 μm. The thickness of a layer of porous material is approximately thesame as the diameter of the particles in the material. In general, athin porous structure with substantially uniform pore sized material mayhelp reduce evaporation resistance and improve the ability to wet theevaporation section.

FIG. 5 is a side view of the heat pipe whose top view is shown in FIG.4. The heat pipe shown in FIGS. 4 and 5 has an approximately rectangularcross-sectional shape. In addition to outer walls 410, wick 420, andevaporator 430, which are discussed above with regard to FIG. 4, theheat pipe may also include a liquid coolant 510 and a vapor space 520.The liquid coolant 210 may include water, methanol, ethanol, acetone,heptane, Freon, or another refrigerant, or a mixture of any of theabove. The liquid coolant may pool on the surface of the evaporator, asillustrated in FIG. 2, and may also permeate wick 420. The liquidcoolant may be evaporated by boiling over the evaporator. In oneembodiment, wick 420 may extend vertically above the evaporator toimprove wetting of evaporator by the liquid coolant. In anotherembodiment, wick 420 may not extend vertically above the evaporator. Insuch an embodiment, however, the amount of coolant 510 should besufficient to ensure continuous wetting of the evaporator.

Vapor space 520 may be located between wick 420 and the top one of outerwalls 410. When liquid coolant 510 is evaporated in the evaporatorsection, the vapor pressure in the evaporator section becomes higherthan that in the condenser section. The pressure difference thus helpstransport vapor to the condenser section of the heat pipe via vaporspace 520 (and possibly also wick 420), where it cools, becomes liquid,and is transported back to the evaporator section by the wick.

Although certain example numerical ranges are given above for thickness,sizes, and values, these ranges are purely exemplary and may varyaccording to design needs. The values given may vary, for example,10-30% above and below the respective endpoints of the ranges givenabove.

Furthermore, although an example embodiment of the present disclosure isdescribed with reference to diagrams in FIGS. 1-5, persons of ordinaryskill in the art will readily appreciate that many other methods ofimplementing the present invention may alternatively be used. Forexample, the order of execution of the functional blocks or processprocedures may be changed, and/or some of the functional blocks orprocess procedures described may be changed, eliminated, or combined.

In the preceding description, various aspects of the present disclosurehave been described. For purposes of explanation, specific numbers,systems and configurations were set forth in order to provide a thoroughunderstanding of the present disclosure. However, it is apparent to oneskilled in the art having the benefit of this disclosure that thepresent disclosure may be practiced without the specific details. Inother instances, well-known features, components, or modules wereomitted, simplified, combined, or split in order not to obscure thepresent disclosure.

While this disclosure has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications of the illustrative embodiments,as well as other embodiments of the disclosure, which are apparent topersons skilled in the art to which the disclosure pertains are deemedto lie within the spirit and scope of the disclosure.

1. A cooling apparatus, comprising: a heat pipe to transfer heat from afirst section to a second section, the heat pipe having an evaporatormade of substantially uniform pore sized material; and a heat exchanger,coupled to the heat pipe, to dissipate heat from the second section ofthe heat pipe.
 2. The cooling apparatus of claim 1, wherein the firstsection comprises an evaporator section in the heat pipe, and the secondsection comprises a condenser section in the heat pipe.
 3. The coolingapparatus of claim 1, wherein the heat pipe comprises: a liquid coolant;an evaporator, when heated, to evaporate at least a portion of theliquid coolant, the evaporator having a thin porous structure made ofsubstantially uniform pore sized material; and a wick to bring the atleast a portion of the liquid coolant to the evaporator.
 4. The coolingapparatus of claim 3, wherein the heat pipe further comprises acontainer to enclose the evaporator, the wick, and the liquid coolant.5. The cooling apparatus of claim 3, wherein the evaporator comprises atone to two layers of porous material with the thickness of a layer beingapproximately the average diameter of particles that make the layer. 6.The cooling apparatus of claim 5, wherein the particles in theevaporator comprises approximately substantially uniformly sizedparticles with an average diameter in an approximate range from 25 μm to150 μm.
 7. The cooling apparatus of claim 3, wherein the wick comprisesa porous structure made of finely porous material with average thicknessin an approximate range from 0.5 mm to 1 mm.
 8. The cooling apparatus ofclaim 3, wherein the porous structure of the evaporator comprisesparticles whose average diameter is larger than the average diameter ofparticles in the porous material in the wick.
 9. The cooling apparatusof claim 3, wherein the average thickness of the porous structure of theevaporator is thinner than the average thickness of the porous structureof the wick.
 10. A heat pipe, comprising: a liquid coolant; anevaporator, when heated, to evaporate at least a portion of the liquidcoolant, the evaporator having a porous structure made of substantiallyuniform pore sized material; and a wick to bring the at least a portionof the liquid coolant to the evaporator.
 11. The heat pipe of claim 10,further comprising a container to enclose the evaporator, the wick, andthe liquid coolant.
 12. The heat pipe of claim 10, wherein theevaporator comprises one to two layers of porous material with thethickness of a layer being approximately an average diameter ofparticles that make the layer.
 13. The heat pipe of claim 12, whereinthe particles in the evaporator comprises approximately uniformly sizedparticles with an average diameter in an approximate range from 25 μm to150 μm.
 14. The heat pipe of claim 10, wherein the wick comprises aporous structure made of finely porous material with average thicknessin an approximate range from 0.5 mm to 1 mm.
 15. The heat pipe of claim10, wherein the porous structure of the evaporator comprises particleswhose average diameter is larger than the average diameter of particlesin the porous material in the wick.
 16. The heat pipe of claim 10,wherein average thickness of the porous structure of the evaporator isthinner than the average thickness of the porous structure of the wick.17. A system, comprising: a heat-generating device; and a cooling systemto dissipate heat generated by the heat-generating device using a heatpipe, the heat pipe having an evaporator made of substantially uniformpore sized material.
 18. The system of claim 17, wherein theheat-generating device comprises an integrated circuit chip.
 19. Thesystem of claim 17, wherein the cooling system comprises: a heat pipe,including an evaporator section and a condenser section, to transferheat from the evaporator section to a condenser section, the heat pipehaving an evaporator made of substantially uniform pore sized material;and a heat exchanger, coupled to the heat pipe, to dissipate heat fromthe condenser section.
 20. The system of claim 19, wherein the heat pipecomprises: a liquid coolant; an evaporator, when heated, to evaporate atleast a portion of the liquid coolant, the evaporator having a porousstructure made of substantially uniform pore sized material; a wick tobring the at least a portion of the liquid coolant to the evaporator,the thickness of the wick being greater on average than the thickness ofthe evaporator; and a container to enclose the evaporator, the wick, andthe liquid coolant.
 21. The system of claim 20, wherein the evaporatorcomprises one to two layers of porous material with the thickness of alayer being approximately the average diameter of particles that makethe layer, the average diameter of particles being in an approximaterange from 25 μm to 150 μm.
 22. The system of claim 20, wherein the wickcomprises a porous structure made of finely porous material with averagethickness in an approximate range from 0.5 mm to 1 mm.
 23. The system ofclaim 20, wherein the porous structure of the evaporator comprisesparticles whose average diameter is larger than average diameter ofparticles in porous material that makes the wick.
 24. The system ofclaim 19, wherein the heat exchanger extracts heat from vapor in thecondenser section to condense the vapor to liquid.
 25. The system ofclaim 24, wherein the heat exchanger comprises a fan to enhance air flowand a plurality of fins to dissipate heat extracted from the vapor toambient air.